Drying of vegetables



March 21, 1944. V 2,344,754

DRYING OF VEGETABLES Filed Aug. 29, 1942 2 Sheets-Sheet 1 .F LTE. 2.

"1555c! LLATCIR IN VEN TOR ATT 012/1553;

March 21, 1944. A. VANG 4 DRYING OF VEGETABLES 2 Sheets-Sheet 2 Filed Aug. 29, 1942 w M W IN VEN TOR.

AZ'ZURAZIX USC/LLATUR Patented Mar. 21, 1944 DRYING OF VEGETABLES Alfred Vang, Newark, N. 1., assignor of one-half to Stevenson, Jordan -& Harrison, Inc., New York, N. Y., a corporation of New York Application August 29, 1942, Serial No. 456,654

4 Claims.

This invention relates to new and useful improvements in the drying of vegetables, fruits, and other food products and more particularly it refers to a method and means forkeeping such articles in vibration by means of audible or supersonic vibrations during the process of drying, thereby assuring more uniform products and better control of the drying process.

In the drying of vegetables, fruits, and other articles of food it has heretofore'been customary to apply steam or direct heat in various forms to efiect the evaporation of the moisture contained in said products and thereby to eliminate the same, making the final product much smaller in size and weight and preventing spoilage due to fermentation, etc. In the process of drying, however, due to the difiiculty of heat transfer into the innermost parts of thearticle to be dried, much spoilage has resulted either from not being able to thoroughly dry the inner fibers because sufficient temperature could not be applied, or from burning on the outside because of too high temperature or because a proper stirring action was impossible. Under such conditions close control of the-moisture content of the article to be dried is very essential and the moisture content must be checked at every step of the process to prevent overdrying or underdrying and to prevent scorching or burning and reduction in nutritive value and loss in vitamins, etc. It is particularly important that the drying temperature should not be too high because this has been shown to cause the loss in vitamin contents, as well as in reduction of nutritive value in general.

In my present invention I eliminate the above mentioned difficulties by several co-acting means designed to provide the best possible drying conditions, as well as to produce the best possible final product. One of the most important im-- provements is to be found in my application of high frequency induction heating, whereby, in fact, I transfer the heat into the inner region of the article to be dried in the form of electromagnetic waves and then generate the heat internally, thereby preventing scorching and burning.

'Another important improvement is my method of stirring the articles to be dried by means of vibrations. Thislatter method insures a uni form heating and thereby completely uniform drying, which results in a homogeneous final product. The induction heating by high frequency electric currents is too well known to need any further detailed explanation. It should only be noted that the heating effect is due to two separate and distinct causes, namely, first the heating from dielectric substances absorbing the high frequency electric field, and'second, the heating from eddy-currents in the article to be dried. The dielectric absorption is normally constant andis dependent upon the wave length of the short wave current applied, as wellasthe nature of the material. The heatingby eddycurrents, however,- will depend upon several factors, such as the amount of moisture present, the amount of mineral or fruit salts, as well as on the presence of any organicacids such as citric acid, etc. The main factor, however, will be the amount of moisture present and'this fact enables me to use this characteristic as-a control medium in controlling the process of drying as hereinafter described.-

Any moisture present will be a conductor for currents and will cause the appearance of eddycurrents introduced by the high frequency electrical field. These eddy-currentswill dissipate themselves in the ohmic resistance of the conducting elementspresent and will thus turn into heat. As the amount of moisture is reduced, however, a smaller and smaller amount of eddycurrent effect will be. resent and the heating due to this will be gradually diminished. This phenomenon enables me to apply a. very effective and simple control of the process because the energy absorbed by a certain batch of material will be gradually diminished down to a predetermined end or final point, where it cannot be reduced any further, except by very prolonged drying and changing of the chemical character of the material. It is, therefore, possible to read the output of the high frequency source, and noting the diminishing amount of power absorbed, the predetermined end point can easily be established. By thus switching off the high frequency current when the end point has been reached, burning and overdrying of the material is prevented. f

To further insure uniformity of heating and homogeneous drying, I apply vibratory energy of a predetermined frequency to the drying mass thereby obtaining'in the same an additional stirring effect and a homogenization of the particles involved. It is obvious that if some oi. the particles of the material to be dried as, for instance, in the case of coffee beans, are stuck together the drying will not be as uniform and eiIective as if the particles were equally separated. It is natural to obtain such separation when I apply vibratoryenergy of a suitable frequency, but it must be noted that this energy is also dissipated in the mass to be dried and will be turned into heat. It is well known that every vibrating sys' tem has what is called a natural or resonant frequency at which point it will vibrate with much greater violence and with less input of power than at any other frequency. I can predetermine the most eflicient frequency by means of variable frequency apparatus, and thereby insure the most eifective use of the vibratory energy. I have found by experience that in vibrating a mass of granular material such as, for instance, coffee beans or similar articles, the beans will be set into a vibratory motion and will bounce against one another in such a manner that there will be a practically constant air space between said beans. The size of the air space and the consistency of same will be dependent upon the mass of the individual bean, as well as the uniformity of the size of the same. Inasmuch as such beans usually come graded to size. the uniformity is very great and a uniform air space between each bean is thereby insured. The same applies to such articles as potatoes, carrots, turnips, etc., which are usually diced into small cubes of exactly uniform size. By thus insuring the presence of a practically constant air space between all the parts of the mass to be dried, a uniform heating and drying. as described supra, will result.

The object of my invention, therefore, is to provide a method and means by which vibratory energy produces a uniform and homogeneous drying of vegetables and related food articles.

Another object of the invention is to provide high frequency induction heating of such frequency that the power absorbed will diminish with the diminishing amount of moisture present, thereby furnishing a means of controlling the same.-

Other objects and advantages of the invention will be apparent during the course of the following description. In the accompanying drawings, forming a part of this specification, and in which like numerals are employed to designate similar parts throughout the same:

Figure 1 is a magnified section through a diced part of a potato before drying,

Figure 2 is a magnified section through the part shown in Figure,1 after drying,

Figure 3 is a graphical representation of the temperature gradient in the drying of a bean or a diced piece such as shown in Figure 1,

Figure 4 is a perspective view of a device embodying my invention,

Figure 5 is a section along the line 5-5 in Figure 4.

Figure 6 is a section along the line 8-4 in Figure 5.

Figure 7 is a section through a mass of small berries or beans before applying the vibratory power; and,

Figure 8 is the same section, as shown in Figure 7, after applying the vibratory power.

In the drawings, wherein for the purpose of illustration, is shown a preferred embodiment of my invention, the numeral iii designates the diced material in Figure 1; and II represents the same material in Figure 2 after drying. Referring particularly to Figures 4. 5, and 6. i2 is a container or kettle in which the material to be dried is placed, and which may be made from glass or other suitable material. The container I2 is closed at its top by means of a cover l3 with a small observation or test window I closed by cover IS. The shaft I6 is rotatably mounted in the cover l3 and extends to a multivaned stirring rotor H. The article to be dried, II, is placed in the container i2 and is rotated by the rotor II. On the outside of the container I2 is placed a winding l9 conducting the high frequency electric heating current, which is connected at the ends 20 and 2| to the winding l8. It is obvious that a switch may be provided for applying this heating current at will, but this is usually included in the high frequency generating apparatus, as is well known in such applications. A control switch for the high frequency current is, therefore, not shown in the drawings. The cover i 3 also carries a ventilating tube 22, through which may be eliminated any vapors which may be created in the container I 2.

Underneath the container l2, as shown in Figures 4 and 6, is placed a vibration exciter 23 working upon the plate 24 cemented in the bottom of the vessel I 2. The vibration exciter 21 is driven by coils 25, which again are fed through wires 26 and 21 from oscillator 28. The plate 2 is placed in the container i2 in such a manner as to act as a membrane for the transmission of any vibrations created by the vibratory exciter 23. The vibration exciter 23 may be a magnetostriction rod or simply a magnetic coil working upon a membrane, all according to the frequency of the vibration desired and according to the amount of power necessary.

In Figure 7 is shown a cross section through a drying mass of material made up of small units 29, such as beans, berries, or the like, before any vibrations have been applied. In Figure 8 the same cross section is shown after the application of vibrations. It will be noted that the vibrations have neatly arranged the units 29 and spaced the same equally from one another.

In Figure 3 is indicated the temperature gradient for the transmission of heat from the outside of an article such as a bean into the inner portions of same. It should be noted that the temperature at the outside skin of the article is very much higher than in the interior of the same. For this reason any drying by means of external heat is liable to scorch and burn the outside of the article without penetrating sufficiently to the interior and thereby leaving the interior with a too high moisture content. It is, therefore, essential that the heat be created in the interior of the article to be dried such as is the case in my present invention, where I utilize high frequency induction and absorption heating. With my method, therefore, it is possible to create an even temperature all the way through the material to be dried and to avoid the steep temperature gradient shown in Figure 3, where the upper end 30 of the curve 3i represents the temperature of the outer skin of a given object to be dried, while the lower end 32 represents the temperature in the interior of same.

The operation of my invention is as follows:

As mentioned supra, the article to be dried is placed in the contained i2 in a suitable quantity and the shaft it with rotor i1 is mechanically rotated to give the drying mass a continuous mechanical stirring. At the same time a high frequency current is sent through the windings is thereby applying high frequency induction and absorption heating to the drying mass i8. It is advisable that the frequency of the high frequency current be selected to give a maximum eddy-current or induction heating effect when the article to be dried is just put in, i. e., when it is still in its wettest stage. By utilizing a frequency of thisnat'ure, the current or power applied to the heating will have what is called a fallingcharacteristic, i. a, it will gradually diminishasthedryin processproceeds. Thisis due to the fact that the diminishing moisture in the article to be dried reduces the eddy-current effect, and after all the moisture is driven out there remains only they absorption heating because no eddy-currents are possible as the conductivity of the material has been eliminated.

This is an important part of my invention because it enables me to control the moisture contents of the drying mass within very close limits.

After having once established the frequency and the suitable input power to start the drying, it is only necessary to watch the falling characteristic of the current input and to switch oi! the same as soon as it reaches a predetermined min- The switching of the high frequency current has not been shown and is not claimed as new. The most advantageous switching is usually done by electronic means and in the present case the electronic switching means could be operated by a low current relay, which would trip the switchingmeans assoonasthecurrenthadbeenreduced to a desired minimum. In this manner the operation of the device can be made entirely automatic and requires attention only during the filling and emptying of the batches.

As mentioned supra, the vibration exciter 23 works upon plate 24 at the bottom of the container if to excite forced vibrations in the same,

. which vibrations again are transmitted upward to the drying mass it. It has also been mentioned supra how the vibrations thus excited willarrangethedryingmass iiinauniform fashion, as indicated in Figure 8, where the units is are equally spaced from one another. As mentioned supra, the determination of the natural frequency of the vibrating system is made by means of variable frequency apparatus for that various changes in the shape, size, and arrangementofpartsmaywellberesorted to, without departing from the spirit of my invention or the scope of the subioined claims.

Having thus described my invention, I claim:

1. A method for drying vegetable particles, or the like, of the character described. consistin in placing the vegetable, or the like particles, in a closed container of imperfect dielectric material which has a small vent for the escape of vapors, subjecting the compass within said container to a high voltage and high frequency electric field for heating the moisture withm and throughout the mass of said particles, and imparting vibrations to said container for vibrating said particles to assume a homogeneous mass within said electric field.

2. A method for drying vegetable particles or the like particles, of the character described, consisting in placing the vegetable or the like particles in a closed container of imperfect dielectric material which has a small vent for the es cape of vapors, stirring said particles for continuously changing their positions, subjecting the compass within said container to a high voltage and high frequency electric field for heating the vmoisture within and throughout the mass.

' of said particles, and imparting vibrations to the purpose of establishing the most efiicient ll 4. An apparatus for drying vegetables, or the frequency. It is well known that a vibrating system has a natural or resonant frequency at which it will vibrate with great intensity and large amplitude with minimum input power. The mechanical vibrations created by the vibration exciter 23 are, of course, applied during the same time as the high frequency heating current is applied to the windings II. The switching of the power delivered from the oscillator 20 to the coils 25 may also, if desired, be controlled by the same means mentioned supra for the control of the high frequency current. I

Itistobeunderstoodthattheformof invention, herewith shown and described. is to betakenasapreferredexsmpleofthssameand said container for vibrating said particles to assueilne a homogeneous mass within said electric n d.

3. An apparatus for drying vegetables, or the like particles, of the character described, comprising a closed imperfect dielectric container, having a small vent for the escape of vapors and adapted to hold said particles, means for sub- Jecting the compass within said container to a high voltage and high frequency electric field for heating the moisture within and throughout the mass of each particle, and means for vibrating said container for vibrating said particles to assume a homogeneous mass within said electric field.

' said-particles for continuously changing their relative positions, means for subjecting the compass within said container to a high voltage and high frequency electric field for heating the moisture within and throughout the mass of each par- 88 ticle, and means for vibrating said container, for

vibr tin saidpar clestoassumeah ousmasswithinsaideleetricneld.

swam TANG. 

